It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country.
The successful development of any medicinal compound relies on specific and potent pharmacological activity combined with efficient delivery of the molecule to its target site. Many potential drugs and medicinal peptides fail to reach the marketplace due to poor bioavailabillity.
Poor oral absorption presents a significant barrier to the clinical success of many drugs, particularly peptides. Drug delivery strategies seek to overcome the physical and chemical properties responsible for this poor bioavailabillity, including molecular size, charge, hydrophilicity, hydrogen bonding potential and enzymatic lability. There are only a few reliable examples of therapeutic levels for peptides and proteins being achieved via the oral route.
A number of approaches have been employed to improve oral bioavailabillity for therapeutic molecules. These include the use of penetration enhancers, which alter membrane permeability non-specifically [Lee, V. H. L.; Yamamoto, A.; Kompella, U. B. Crit. Rev. Ther. Drug Carrier Syst., 1991, 8, 91-192.], the use of drug delivery systems such as liposomes, microparticles and microemulsion systems which protect the drug from the environment, and the use of prodrugs which modify the drug molecule itself to impart the desired physicochemical properties.
It is believed that the more lipophilic the molecule, the faster and more completely a drug molecule crosses the intestinal barrier. There is a danger, however, of making a drug too lipophilic for epithelial transport. Results suggest that there is a degree of lipophilicity which is “optimal” for absorption. Highly lipophilic drugs suffer from poor aqueous solubility, which is also necessary for successful oral uptake.
Occasionally hydrophilic drug molecules show unexpectedly high rates of oral absorption. Two mechanisms have been proposed to explain this effect. Active transport systems can be accessed by some molecules resulting in the “pumping” of hydrophilic molecules into the body. Alternatively, ion pair transport has been proposed to explain the unexpected absorption of highly hydrophilic drugs such as the tetracyclines, which are charged over the range of physiological conditions, and are generally lipid-insoluble [Meyer, J. D.; Manning, M. C.; Hydrophobic Ion Pairing: Altering the Solubility Properties of Biomolecules. Pharm. Res., 1998, 15, 188-193]. The interaction of such drugs with endogenous counter-ions in effect “buries” the charge within the ion pair, forming a neutral species, which may be able to traverse the epithelium. Hydrophobic ion pairing represents an inexpensive and reversible means by which to modify the physicochemical properties of a drug without the need for irreversible chemical modification [Neubert, R. Ion Pair Transport Across Membranes. Pharm. Res., 1989, 6, 743-747].
The ability to form an ion pair and the success of improving transport by this approach depends very greatly on the physicochemical properties of both the drug and the counter-ion.
An ion pair can be defined as a neutral species formed by electrostatic attraction between oppositely charged ions in solution, which are often sufficiently lipophilic to dissolve in non-aqueous solvents [Quintanar-Guerrero, D.; Allemann, E.; Fessi, H.; Doelker, E. Applications of the Ion-Pair Concept to Hydrophilic Substances with Special Emphasis on Peptides. Pharm. Res., 1997, 14, 119-127.].
The lipophilicity of hydrophilic ionised drugs can be increased by ion pair formation with lipophilic counter-ions such as hexylsalicylate or decylsulphate. It appears that ion pair formation only affects the partition and transport of hydrophilic drugs which are charged in the media in which ion pairing takes place.
Although counter-ions such as alkylsulphates, trichloroacetate and alkylcarbonates have been used for ion pairing, it has been suggested that these counter-ions are too irritant to the gut at the required dosages [Neubert, et al op. cit.]. Pharm. Res., 1989, 6, 743-747 and references here-in]. Counter-ions need to have the following properties: high lipophilicity, sufficient solubility, physiological compatibility and metabolic stability. Suitable counter-ions include alkanoic acids [Green, P. G.; Hadgraft, J. Int. J. Pharm., 1987, 37, 251-255] and alkylated salicylic acids [Neubert, R. Ion Pair Transport Across Membranes. Pharm. Res., 1989, 6, 743-747].
It was initially supposed that the two components of an ion pair traverse lipid membranes at an equimolar ratio. However, the mechanism may be more complex. Experiments based on lipophilic counter-ions for cationic drug transport showed that the counter-ions accumulated in the membrane, and that, as a result, more hydrophilic drug molecules than counter-ions were transported. Transport of the complete ion pair was also demonstrated. (Neubert et al. 1989 op. cit.]. A similar mechanism has been proposed for the transport of anionic drugs [Hadgraft, J.; Wotton, P. K.; Walters, K. A. J. Pharm. Pharmacol., 1985, 37, 757-727].
The approaches discussed thus far are based on increasing lipophilicity for enhanced transport by passive diffusion via the transcellular pathway. An alternative strategy is to exploit the numerous active transport mechanisms present in the gastrointestinal mucosa. Strategies have been designed to improve the bioavailability of poorly absorbed drugs and peptides so that they can be absorbed by specialised intestinal transporters.
Conjugation of a saccharide moiety to a poorly absorbed drug improves its solubility in aqueous media due to the poly-hydroxyl nature of sugars. In addition, sugar conjugation may allow passage of the sugar-drug conjugate across the gut via the SGLT-1 glucose transporter [Gould, G. W.; Holman G. D. The Glucose transporter family: structure, function and tissue-specific expression. Biochem. J., 1993, 295, 329-341]. The effectiveness of this approach has been demonstrated by conjugation of a glucose derivative to a tetrapeptide not normally transported by PepT1 [Nomoto, M.; Yamada, K.; Haga, M.; Hayashi, M. Improvement of Intestinal Absorption of Peptide Drugs by Glycosylation: Transport by the Sodium Ion-Dependent D-Glucose Transporter. J. Phar. Sci., 1998, 87, 326-332]. Interestingly, the configuration at the anomeric centre of the sugar was found to affect the rate of transport: A β-anomeric linkage was preferred over the α-configuration. Subsequently, further evidence was presented for glycosides of paracetamol [Mizuma, T.; Nagamine, Y.; Dobashi, A.; Awazu, S. Factors that cause the β-anomeric preference of Na+/glucose cotransporter for intestinal transport of monosaccharides conjugates. Biochim. Biophys. Acta, 1998, 1381, 340-346]. Glucose conjugates were transported more efficiently than galactose conjugates, with the β-trans-anomeric configuration preferred in both cases. Galactose conjugates with the α-cis-configuration were not transported at all.
We have previously demonstrated the utility of conjugating lipoamino acids or lipoaminosaccharide constructs to drug molecules through a covalent bond (International Patent Application No. PCT/AU01/01313 filed 18 Oct. 2001; Toth et al., 1993; Toth and Gibbons, British Patent Application No. 9215780.9 (24 Jul. 1992); Toth and Gibbons, European Patent Application No. 93917902.4). These compounds provide an excellent delivery system, but require the chemical conjugation of the drug molecule to the delivery system.
We now propose the use of lipoamino acids and lipoaminosaccharide conjugates as an ionic delivery system in which the drug molecule and the delivery system form an ionic complex. This system does not require the chemical conjugation of the drug molecule, and therefore will not alter the pharmacological properties of the drug molecule. In addition, this method of delivery can be used to target either passive or active transport mechanisms. The proposed delivery system is readily optimised for hydrophilic drug molecules, peptides and proteins, and offers significant benefits in terms of regulatory approval. We believe that this is the first example of the use of non-covalently linked lipoamino acids and lipoamino saccharides for drug delivery.